The present invention relates to a method for controlling an anti-lock brake system for automotive vehicles with all-wheel drive. Electric signals representative of the rotational behavior of the vehicle wheels are generated, wherefrom, after the electronic processing and logic combining of said signals, braking pressure control signals are derived which serve to reduce the braking pressure or to keep it constant and to re-increase it at the appropriate time upon the occurrence of a tendency to lock. Wheel slip and wheel acceleration as well as wheel deceleration are evaluated as control criteria. The individual wheel speed is compared with a vehicle reference speed which is formed in consideration of the rotational behavior of all wheels and which serves as a reference value for the control of the braking pressure of the single wheels.
To control wheel slip during a braking action with an anti-lock, that means slip-controlled, brake system, there is a need to measure the speed and speed variations of the controlled wheels and to compare these values with a suitable reference value. This reference value must display in what fashion the braking pressure must be varied, for instance by means of an electronic control circuitry, in order to prevent locking of the wheels and to thereby maintain driving stability and steerability without making the stopping distance longer. In known slip-controlled brake systems, the so-termed vehicle reference speed serves as a reference value which, in the ideal case, represents the vehicle speed in consideration of the optimum wheel slip.
To determine the actual vehicle speed and the vehicle reference speed from the rotational behavior of the individual wheels affords difficulties during a braking action because slip occurs at every wheel and because road conditions, the instantaneous load of the single wheels, discrepancies due to conering etc. become part of the measurements. To diminish these difficulties, it is known to determine the reference speed by means of a wheel which is freely running even during the braking action, or by an additional wheel, or by a wheel which temporarily is not braked, or even with the aid of a Doppler radar system. These measures have disadvantages for various reasons.
In actual practice, the vehicle reference speed is determined by logically combining the rotational behavior of the individual wheels. In known circuits, this reference value is determined by the instantaneously fastest or second fastest wheel, depending on the situation, while in addition a limitation to the deceleration or acceleration of the vehicle which is maximum possible for physical reasons is performed in various situations.
It has also been proposed (German printed and published patent application 33 45 730) to evaluate the wheel rotational behavior of each wheel in comparison to a wheel-related vehicle reference speed according to several preselected criteria and to subsequently divide into reference-forming phases, in which in each case a specific variation of the reference value is preset.
In vehicles having but one driven axle, spinning of the driven wheels due to the moment of inertia of the engine with the engine in mesh and on slippery roads or as a consequence of too high driving force can be recognized relatively easily in most situations by comparison with the rotational behavior of the non-driven wheels. In vehicle with all-wheel drive, to which the present invention relates, additional difficulties result from coupling of the wheels by way of the drive shaft. This is because on surfaces with a low frictional value, when the differences in the road torque at the individual wheels become less than the coupling torques of the wheels among themselves, a slowly increasing positive slip may develop synchronously at all wheels which cannot be distinguished from acceleration of the vehicle on roads with high frictional value alone by virtue of measurement of the wheel rotational behavior and logic combining of the individual wheel information. In this case, the vehicle reference speed may rise far beyond the vehicle speed. If a braking action were initiated, the electronics would detect a great difference between the vehicle wheels, which in reality are running stably, and the excessive vehicle reference speed and consequently would bring about a control action. That is, it would arrange for the pressure to be maintained constant or even decreased. Hence follows that spinning of all four wheels leads to a too high vehicle reference speed, having as a consequence that, even if the wheels run stably again afterwards, the braking pressure will be reduced too much for as long as until the reference value has dropped to attain the vehicle speed again. Beside a braking action, a wrong reference speed can have detrimental effects, it can cause an unwanted valve actuation, for instance.
Also known is a circuit configuration to prevent the wheel speed form exceeding the vehicle speed in vehicles with anti-lock brake systems (German Pat. No. 27 40 419). The circuit configuration as disclosed in this patent comprises as threshold value stage to generate positive slip-control signals serving to actuate an inhibiting circuit which prevents the reference speed from rising in excess of the vehicle speed in each and every situation. In vehicles with all-wheel drive in which the speed of all wheels synchoronously may rise to exceed the vehicle speed, such a known circuit configuration will fail.
In German patent specification 23 03 660 an electronic circuit configuration is disclosed for an anti-lock vehicle brake system which serves to decrease the vehicle reference speed by special means after a quick speeding up of the driven wheels. To this end, a transistor in combination with a diode circuit is used permitting to quickly discharge a capacitor, the voltage of which corresponds to the reference value. Such a circuit is also principally inappropriate for an all-wheel drive for the reasons explained above.
Therefore it is an object of the present invention to overcome the described shortcomings of known circuit configurations and to develop a method for controlling an anti-lock brake system which under all conditions ensures in an all-wheel drive that spinning of the wheels is recognized and unwelcome braking pressure reduction in consequence of such a situation is avoided.